1. Field of the Invention
The present invention relates to electrophotographic printing using diffusive light sources and document image capturing or scanning.
2. Description of the Related Art
There are several types of electronic printing devices, for example, wire dot printers, electrophotographic printers, and inkjet printers. Currently, electrophotography and inkjet are two leading electronic printing systems for use in office, home, small office-home office (SOHO) or industrial environments. Electrophotographic printing devices are of relatively faster printing speed and are capable of massive print jobs, while inkjet printers are for relatively slower and smaller print jobs but provide very high printing quality.
Electrophotography is a method of printing electronic information using a series of basic steps: exposure, development, and image transfer, just like photography. A laser printer is a typical commercial machine making use of electrophotography. The basic configuration of a conventional laser printer is depicted in FIG. 20.
A laser printer includes a laser 1 as a light source, an optical system including a polygon mirror 3 and an f-theta lens 5, an organic photo conductor (OPC) drum 7, a developing station 9 including coloring toner particles 11 and paper 13. The laser 1 is a single light source and emits a coherent laser beam that strikes a surface of polygon mirror 3. Polygon mirror 3 rotates at a predetermined speed to deflect the incoming beam to OPC drum 7 through f-theta lens 5 to raster scan the electronic data line-by-line. OPC drum 7 is electrically charged by an electric charger 2 located at an up-rotation position before the laser beam strikes the surface of OPC drum 7. Upon illumination of the laser beam on the OPC drum surface, the illuminated portion changes to be neutralized due to a mechanism of organic photo conduction wherein electric current is created by a photo-conducting effect. This process is called exposure, and the charges after exposure make an image corresponding to the original electronic data. This image is called a latent image Then, coloring toner particles, which have opposite polarity of electric charge compared to the latent image, are attracted to OPC drum 7 by electrostatic forces to create a real image; this is called the development stage. After development, the real image with toner particles is transferred onto a medium such as paper to finalize a print operation.
Recently, laser printers have become much smaller as a result of reduction of the number of parts and compact design. Another effort to reduce the size of electrophotographic printers is to replace the laser source with a line of light sources commensurate with a full page width of a paper sheet. This use of a linear array of light sources leads to elimination of the optical unit, which includes polygon mirror 3 and f-theta lens 5 and which occupies a relatively large space and volume.
The LED printer is this kind of new electrophographic printer in which an LED array head commensurate with a full width of a paper sheet is used as the light source and therefore the polygon mirror and f-theta lens are eliminated. An example of such a printer is US 2004-0169718A1. As an alternative light source for the electrophotographic printer, an organic electro-luminescence (OEL) light source may be used. JP 2004-195676A2 (corresponding to US 2004-0233271A1) discloses an OEL print head that includes an array of electro-luminescent (EL) light sources and an array of individual small lenses.
These EL light sources are formed on a planar substrate by a common technique using general semiconductor processes with relatively cheap cost in any length of array. But it has been difficult to extract light efficiently from this type of light emitting device because the EL source is a planar light source with Lambertian light emitting characteristics rather than a point light source with limited emission angle like a laser. This type of light source is sometimes called a diffusive light source.
In general, a diffusive light source is a planar light source that has a wide range of emission angles. Some types of diffusive light sources emit light subject to the Lambertian law and hence are called Lambertian light sources. The radiance (power per unit square meter per unit solid angle) of a Lambertian light source at an angle from the normal with respect to the plane of light source I(θ) is expressed as:I(θ)=I0 cos θ(W/m2·sr),  (1)where I0 denotes central radiance at the orthogonal direction with respect to the plane of the light source. This formula means that a Lambertian light source emits light in all directions in a hemisphere around the light source. For example, the intensity of the light at 60 degrees from the orthogonal direction is still one half of that at the orthogonal direction. This property of a Lambertian light source has given rise to a problem that “modulation” is likely to be very poor compared to a laser light source. Modulation Mod is, in the context of electrophotography, defined as a measure of how the images on the OPC drum from two light sources set apart at a certain distance on the light source array are distinguishable (see FIG. 21), and is expressed in a formula as:Mod=(Imax−Imin)/(Imax+Imin),  (2)where Imax and Imin are the maximum intensity and minimum intensity, respectively, and P1 and P2 are the positions of two separate light sources with separation |P2−P1|.
Modulation determines the resolution of the image printed. When a Lambertian light source is used in an array of light sources, modulation is quite poor because the Lambertian light source has such a wide emission angle that a considerable amount of stray light from adjacent light sources tends to interfere with the other light source. This has been a problem when diffusive light sources are employed.
Another problem with a Lambertian light source is efficiency of usage of light power. The total power in a cone with half cone angle θ0 around the normal of the plane of light source P(θ0) is calculated by integrating Equation (1) to give:P(θ0)=πI0 sin2 θ0(W/m2)  (3)This formula indicates that a Lambertian light source emits only one half of all power in 45 degrees of half cone angle around the normal. This also means that the rest of the half power of light is outside the cone. In conjunction with the difficulty of achieving a good modulation, efficient use of Lambertian light is also a serious problem to be solved.
There are only a few prior art references that even address this problem. JP 2004-195676 (corresponding to US 2004-0233271A1) and JP 2004-195677 (also corresponding to US 2004-0233271A1) are devoted to improving the extraction efficiency of light power from EL sources, where individual micro ball lenses are placed mostly contacting with each of the EL sources to extract the light as much as possible and refract it towards the image plane (OPC drum surface in case of electrophotography). FIG. 22 depicts an arrangement employing micro ball lenses. This technique includes two effects, i.e., (1) capturing the light and (2) holding the light. By placing a lens very close to a light source, the lens can capture more light. This follows from the fact that the solid angle from the light source that can be captured by a lens in front of the light source progressively increases as the distance between the light source and the first surface of the lens decreases. In this way, any lens works well to capture more light if it is positioned close to or in contact with a light source.
In addition to this effect, a ball lens has characteristics that enable it to hold the light inside the lens because of the total internal reflection that takes place when light rays are introduced inside the ball lens. In this way, a ball lens might work well in terms of efficient extraction of the light from diffuse light sources.
However, a ball lens is not suitable at all for transferring the light that is captured inside the ball. It is very hard to get a good image on an image plane without suffering aberration by using a ball lens, namely, all the light rays collected hardly gather into one point on an image plane. That is, a poorly converged/spread image is obtained, and this brings a problem of poor modulation. This problem becomes serious when there is a demand of increase of focus depth, namely, the distance between the lens and the image plane. This demand that the OPC drum should be kept at a certain distance from the light source occurs for various reasons, for example, the tolerance of the position of the surface of the OPC drum when it rotates and so forth. To meet this demand, a ball lens is not an option for achieving a well-designed surface to get a good image since the surface of a ball lens is just a spherical surface. In order to eliminate as much aberration as possible, the surface of a lens should have a complex geometry. It is quite difficult to make such a surface on a ball lens.
Another problem with a ball lens is that a ball lens can suffer chromatic aberration. As an EL source has a broad wavelength distribution, chromatic aberration also leads to a poor image quality and therefore leads to poor modulation and loss of light. In order to eliminate chromatic aberration, there are various techniques, such as to adapt a lens with an aspheric surface or an achromatic lens; however, this can not be applied to a ball lens. Thus, the technique using a ball lens is not suitable for transferring the light with a good modulation without aberration although it is easy to collect light.
Another technique for applying diffuse light sources to electrophotography is to combine the light sources with a coupling optical system, which is something like a fiber. This technique is used in an LED printer, where a so-called GRIN (Gradient Index) lens is employed in an array form together with an array of LED sources. As examples of an LED printer, there are K. Matsuda, et al., “High-Speed Color Printer Engine,” OKI Technical Review, Vol. 70, No. 2, April, 2003 and U.S. Pat. No. 6,646,670B2. A GRIN lens is sold by many companies, for example, Nippon Sheet Glass Co. Ltd., Tokyo, Japan. In this example, a GRIN lens performs a function of collecting and transferring the light for multiple LED sources. Therefore, the number of GRIN lenses is less than that of the LED sources, in which case the GRIN lens can be erected into an image system with the same magnification. A problem of this system is that the coupling efficiency is low and not cost effective, while transfer characteristics are comparatively good.